1. Field of the Invention
The present invention relates to an air-fuel mixture ratio control device adapted to control the air-fuel ratio of mixture in accordance with a signal derived from an exhaust gas sensor to maintain the air-fuel ratio at a desired level.
2. Description of the Prior Art
FIG. 1 exemplarily shows an arrangement of conventional air-fuel ratio control system. A carburetor 2 disposed at the upstream side of the engine body 1 is provided with a pick-up port 3 for bleed air for controlling main fuel system and a pick-up port 4 of bleed air for controlling slow fuel system. An oxygen sensor 6 is disposed at the upstream side of a tertiary catalyst 7 and is adapted to deliver a signal to a control circuit 9 which in turn produces a control signal for actuating an actuator 8. A reference numeral 5 designates an air cleaner, while a reference numeral 10 denotes a battery. A cooling water sensor is designated at a reference numeral 11. The actuator 8 is adapted to adjust the amounts of bleed air at the pick-up ports 3,4 thereby to vary and adjust the air-fuel ratio. The actuator is usually constituted by a step motor adapted to vary the areas of openings of the air bleed passages. A certain time length is required from the formation of the mixture in the carburetor to the arrival of the exhaust gas at the exhaust gas sensor via the step of combustion in the engine. This time length is determined by various factors such as engine speed, intake air flow rate and so forth. In consequence, there is a time lag of detection of the air fuel ratio by the exhaust gas sensor after the formation of the mixture. This constitutes one of the reasons of disturbance of stability of the control system. Therefore, in the conventional system of the kind described, a proportional integrating control in which proportional control and integrating control are combined is adopted to ensure the stability and responsive characteristic of the control system.
In a system incorporating a step motor as the actuator, a clock signal of high speed is delivered to the driving circuit of the step motor only for a predetermined time after the inversion of the output signal from the oxygen sensor, thereby to drive the motor at a high speed to obtain a step operation to achieve the proportional integrating control. In this case, it is necessary to prepare two kinds of clock signal, i.e. a low speed clock of several to several tens of Hertz for integrating control and a clock of several tens to several hundreds of Hertz for skipping operation at a comparatively high speed, and to switch the clock at each time of inversion of the oxygen sensor signal to send the same to the driving circuit. In some cases, two or more clocks of different frequencies are used for the integrating action, due to difference in factors such as rate of air intaking into the engine, engine speed and the amount of change of engine speed. If these clock signals have to be obtained by a demultiplication of the output from a single oscillator, it is necessary to make the oscillator have a considerably high frequency and, in addition, a multiplicity of demultipliers is required which impractically complicates the device. To avoid this, it is a common measure to use two or more independent demultipliers and to switch the output from these demultipliers by means of a gate.
FIG. 2 shows an example of a conventional control circuit. In this control circuit, a high speed clock signal is produced by a circuit including inverters 950,951, resistors R17 and R18, and a capacitor C15, while a low speed clock signal is formed by a circuit including inverters 952,953, resistors R19,R20 and a capacitor C16. A switching between the high speed clock signal and the low speed clock signal is made by the output from a monostable multivibrator which acts at each time of inversion of the signal from the oxygen sensor, and the selected clock signal is delivered to a distribution energizing circuit 962. The monostable multivibrator includes NOR 958, capacitor C14, resistor R16 and an inverter 956. In this state, signals are formed at every part of the circuit as shown in FIG. 3. When the output from the oxygen sensor is changed to invert the direction of control, the clock is switched to the high-speed clock signal for skipping. At this time, there is a possibility that clock of very short period is formed depending on the timing. In this case, it is not possible to obtain the correct operation because the step motor cannot correctly respond. Thus, there is a fear that the required amount of skip cannot be achieved or the inversion is failed to permit the movement in the direction opposite to the desired direction. In addition, when the signal from the oxygen sensor is changed to cause an inversion during the high-speed skipping, an overshoot is caused due to the inertia of the rotor or the like reason resulting in a response failure even though the frequency is within the region of the self-starting frequency. An example of such a mal-functioning is shown by a broken-line arrow in the diagram of the step motor operation in FIG. 3. If the step motor fails to execute the predetermined task, it is quite difficult to achieve the precise air-fuel ratio control, so that the air-fuel ratio of the mixture undesirably comes out of the range suitable for the effective functioning of the catalyst to permit the nonxious exhaust gas components to be released to the atmosphere without being treated.